Joichi Nishi is a middle school student and one of the more experienced participants in the Gantz scenario, having been brought into the room over a year before the main characters, Kei Kurono and Masaru Kato. His death occurred while fleeing from police officers after an incident of shoplifting, which led him to fall from the roof of an apartment building. This followed a difficult home life, as his mother had died by suicide, an event that appears to have profoundly affected him. Prior to his own death, he exhibited disturbing behavior, such as killing animals.

Nishi is characterized by a cold, cynical, and deeply antisocial personality. He maintains a calm and collected demeanor in almost all situations, speaking in a blunt and often scathing tone towards others. His worldview is rooted in a form of social Darwinism, believing that only the strong deserve to survive and showing a complete lack of concern for the welfare of other people. He frequently states that he neither needs nor cares about others, viewing them as either obstacles or useful tools for his own survival. He derives a kind of sadistic enjoyment from watching others die and has deliberately misled new participants about the true nature of the missions for his own amusement. Despite this cold exterior, he displays a notable contradiction: in moments of genuine, life-threatening danger, he often panics and cries out for his mother, revealing a deep-seated fear and vulnerability.

Initially, his primary motivation is to earn one hundred points from Gantz in order to win his freedom from the game, believing he can accomplish this on his own. He is also motivated by a deep-seated hatred for society, which he feels has rejected and bullied him. This culminates in a massacre of his own classmates, from which he spares only one girl who had shown him kindness. As the story progresses and he learns of a coming global catastrophe, his goal shifts from personal freedom to accumulating power and weaponry to face this threat. In his final moments, he becomes somewhat megalomaniacal, entertaining the idea that defeating the alien invaders could make him the ruler of the world.

In the structure of the Gantz game, Nishi acts as a veteran anti-mentor. While he possesses a great deal of knowledge about the system, he refuses to share it freely, offering only cryptic or deceptive information. He is initially killed during the mission against the Tanaka alien, where his overconfidence leads to a fatal confrontation. However, he is later resurrected by another player at the request of Kei Kurono, who hopes to use Nishi's knowledge for the team's benefit. After his revival, he remains aloof and uncooperative for the most part, though he does eventually reveal the existence of the doomsday countdown. He meets his final end while piloting a giant robot in a large-scale battle, where his machine is destroyed after being abandoned by an ally.

Nishi has a few key relationships that reveal different facets of his character. He has a mutual animosity and rivalry with Kei Kurono, particularly after Kurono surpasses him. He shows a measure of respect for another powerful and violent player, Shion Izumi, with whom he shares a past as teammates and can hold a cordial conversation. Contrary to his usual behavior, he makes an unexpected choice to save a young boy named Takeshi from an explosion, a moment of compassion that surprises even himself. His actions are also informed by his past bullying; he specifically targets his classmates for death but spares the one girl who had a crush on him.

Over the course of the story, Nishi undergoes a subtle development. While he never becomes a heroic figure, he is less openly hostile towards the main group after his resurrection. He becomes more focused on the larger threat as his primary motivation, and his exceptional knowledge of the Gantz system becomes a more defined trait, culminating in him hacking the system to deploy himself into combat in a giant robot. His skills prove to be considerable, as he consistently achieves high point scores in missions after his return, demonstrating his proficiency in combat when he chooses to engage.

Nishi's notable abilities are not based on physical strength but on his strategic mind and mastery of the technology provided. His primary tactic is to use the Gantz controller to activate his suit's camouflage function, turning invisible and stalking his targets. He is a proficient user of various Gantz weapons, favoring the Y-gun, which binds targets in wire for transport. He is also one of the few characters shown to have the technical aptitude to hack and manipulate the Gantz system itself, allowing him to access restricted functions and equipment like the giant robot.